1. Field of the Invention
The present invention relates generally to coal material having a small particle size diameter and, in particular, to a method and process for providing a controlled batch of micrometer-sized or nanometer-sized coal material, which exhibits or demonstrates a specified and desired physical or chemical property within a specified range.
2. Description of Related Art
Coal is composed of a complex, heterogeneous mixture of organic and inorganic components that vary in shape, size, and composition depending on the nature of the vegetation from which they were derived, the environment in which they were deposited, and the chemical and physical processes that occurred after burial. As illustrated in Table 2, and according to the prior art, finely sized or pulverized anthracite and other coals are being used in fuel and non-fuel applications, including applications that use these coal materials as precursor particles for the production of high value-added carbon products. These carbon products, however, have minimal or no requirements directed to the exact physical and chemical properties, such as: (1) Particle Size and Distribution (PSD); (2) Particle Shape; (3) Specific Surface Area; and (4) Bulk Purity. Such application needs have been met with little or no success through the prior art coals offered.
TABLE 1United States Sieve Series and Tyler EquivalentsU.S. Sieve No.Tyler Sieve No.DesignationDesignationInchesmmNo. 4  4 Mesh0.1854.699No. 8  8 Mesh0.0932.362No. 10 9 Mesh0.07872.0No. 1210 Mesh0.0651.651No. 1412 Mesh0.05551.4No. 1614 Mesh0.0461.168No. 2020 Mesh0.03280.833No. 3028 Mesh0.02320.589No. 4035 Mesh0.01640.417
Prior art powdered coal materials are being produced and applied with physical dimensions based only on a top size (and infrequently, a bottom size) specification in the mesh size region. The particle size of granular and powdered carbon is typically described by the mesh size through which it can pass. Industry standards use the United States Sieve designation to denote particle size, and the two sieve sizes describe the maximum percentage size and minimum percentage size of the bulk of the material. See Table 1. For example, an activated carbon sized 8×30 contains granules that pass a No. 8 sieve, but not a No. 30 sieve. A 3 mm pellet corresponds to 4×8 granules, while 4 mm pellets correspond to 4×6 granules.
Currently, pulverized coals are only described by the top mesh sieve through which it can pass, for example a “70% minus 200 Mesh (74μ)” designation is used to describe a powder which 70% passes through a Tyler sieve marked 200 mesh, therefore placing negligible restrictions on the number or volume of finest and coarsest particles within the distribution. Such a designation and composition is illustrated in Table 2.
TABLE 2Average Analysis of Pulverized, Impounded and CWSF Coal MaterialsPC, Feedstock andCWSFCWSFCWSF ParametersPCImpounded Coal FinesDay 1Day 2Moisture1.6N.A.a52.151.2(As Received)bAsh14.241.39.99.9Volatile Matter22.622.129.129.3Fixed Carbon63.236.661.160.8Carbon74.648.777.677.6Hydrogen4.33.44.74.7Nitrogen1.30.91.31.3Sulfur1.71.11.41.4Oxygen3.94.65.25.2Higher Heating 12,7377,55413,48613,487Value (Btu/lb)Solids LoadingN.A.N.A.47.948.8(wt. %)cApparent ViscosityN.A.N.A.8099(@ 100 s−1)dpHeN.A.N.A.6.86.5Particle sizeDistribution (μm)f99.8% passing190-245190254293D(v, 0.9) 79-10587117142D(v, 0.5)31-39162930D(v, 0.1)7-8366aNot applicablebAll values reported on a weight %, dry basis unless noted otherwise.cDetermined using an AVC-80 CEM Microwave Moisture AnalyzerdDetermined using a Bohlin Visco 88BV ViscometereDetermined using an Orion Model 420A pH MeterfDetermined using a Malvern Series 2600C Droplet and Particle SizerFrom: Bruce G. Miller, Sharon F. Miller, Joel L. Morrison, and Alan W Scaroni (1997)
Another property to be considered with respect to coal materials is the surface area of the particles. Many of the most popular methods for determining the surface area of powders and porous materials depend on the measurement of adsorption. It was the advent of gas adsorption for surface area determination that had replaced the practice of radioactive indicators and dye adsorption. The first significant advances in the development of gas adsorption technique were made by Brunauer and Emmett, and their work paved the way for the development of the Brunauer-Emmett-Teller (BET) theory in 1938.
Yet another property to be considered is bulk purity, a global property for a population of particles. Bulk purity represents the absence of non-coal or carbon material extrinsic of the particle. Extrinsic impurities may be either organic or inorganic in nature, and their origin may have been from the incoming feed material or during processing and handling.
Traditionally, measurements have been performed in order to rank different coal types. Primarily these measurements were taken to ascertain moisture, volatile matter, fixed carbon, ash and heating value of the subject material. Today, detailed chemical composition measurements can help to design feed material blends for both fuel and non-fuel uses such as Syn-Gas generation, activated carbons, and precursor materials for high value carbon materials such as molded or extruded graphite, binder pitch, ultra-capacitors, etc.
Current art classifies coals according to degree of coalification. The degree of coalification changes depending on factors of pressure, heat and time. As a result, various kinds of coals are generated. These coals have specific properties and are classified for their effective use by rank, a measure of the degree of coalification. In the United States, the American Society for Testing and Materials (ASTM) classifies coal into four classes: lignite, sub-bituminous, bituminous and anthracite.
Table 2 represents the proximate and ultimate analysis of pulverized coal material, and further illustrates the limited information currently required to classify coals. However, this type of information, including the coal's heating value, serves as an adequate yardstick for comparing pulverized materials for fuel use applications only.
For the purposes of this description, and as is known in the art, activated anthracite is also considered activated carbon, such as activated charcoal; active carbon; active charcoal; amorphous carbon; bone black; bone coal; channel black; charcoal; decolorizing carbon; lamp black, etc. Activated carbon is a form of carbon arranged in a quasi-graphitic form and having a small particle size. In particular, this material is a solid, porous, black carbonaceous material and is tasteless. Further, activated carbon is distinguished from elemental carbon by the presence of non-carbon impurities and the oxidation of the carbon surface.
Activated carbon is manufactured in a variety of processes. According to the prior art, activated carbon can be prepared from a large number of sources by a wide variety of methods. The Merck Index divides these into four basic forms: Animal charcoal is obtained by charring bones, meat, blood, etc.; Gas black, furnace black, or channel black is obtained by the incomplete combustion of natural gas; Lamp black is obtained by burning various fats, oils, resins, etc., and Activated charcoal is prepared from wood and vegetables. Activated carbon can be produced from a number of agricultural commodities, such as hardwoods, grain hulls, corn cobs, and nut shells. Steam activation can also be used with food-grade carbonaceous material.
The activation process may also employ an acid treatment. For example, pecan shells can be activated by treatment with hydrochloric acid, then heated in an electric furnace for four hours at 800-1,000° C. in an atmosphere of carbon dioxide. Among the other raw materials used as precursors to make activated carbon are sawdust, peat, lignite, coal, cellulose residues, petroleum coke, spent ion exchange resins, such as styrene-divinyl benzene polymers and phenol-formaldehyde resins, old automobile tires and sewage sludge. Various binding agents may be added to improve the structure. Commercial sources appear to be made from a variety of precursors, activating agents, and binders.
Any given carbon sources may be prepared, treated or manufactured by a wide variety of methods. These may or may not involve synthetic acids, bases, and other substances in a stream of activating gases such as steam (H2O), nitrogen (N2) or carbon dioxide (CO2). Yields and quality can be improved by the removal of moisture. Microwaves can be used to pyrolize the carbon source. Lignite and peat are made into activated charcoal by low-temperature charring, followed by treatment with either superheated steam or potassium hydroxide. Carbon can be made into a cation-exchange resin by sulfonation, or by nitration and reduction, and treatment of low-rank coal with ethylene dichloride and ammonia makes activated carbon an anion exchange resin. Some processes treat carbonaceous matter with phosphoric acid and/or zinc chloride, with the resulting mixture carbonized at an elevated temperature, followed by the removal of the chemical activating agent by water washing. Some activated carbon can be recycled, reactivated, or regenerated from spent activated carbon.
Activated carbon may be used in a variety of specialized applications, for example: as a decolorizing agent; a taste- and odor-removing agent; and a purification agent in food processing. Food and beverage production accounts for only about 6% of the market for liquid-phase activated carbon. Of this, the greatest use is decoloring sugar. More recent applications have enabled the production of xylose and its derivatives from complex cellulose sources via fermentation and activated charcoal. Activated carbon remains the most common method used to de-color vinegar, and can also be used to remove ethylene from fruit storage facilities, particularly if brominated.
The primary use for activated carbon is the treatment of water, including potable water (24% of all use); wastewater (21%) and groundwater remediation (4%), which accounts for approximately half of all the use in the United States. These are indirectly related to organic production, because disinfected water filtered through activated carbon is a common food ingredient. Non-agricultural ingredients, such as enzymes, are also often purified by the use of activated carbon. Both can result in products processed by activated charcoal used to process food and beverages.
Activated charcoal also has non-food uses related to the production and consumption of agricultural commodities. For example, activated charcoal is used to filter tobacco smoke. There are also a number of applications related to purification in the clothing, textile, personal care, cosmetics, and pharmaceutical industries. Activated carbon also has a broad range of applications outside of food processing, such as in veterinary and analogous medical applications, such as detoxification. Activated charcoal is used in agriculture as a soil amendment (e.g., alkali-treated humates and humic acid derivatives), and as a component of nursery or transplant media, as well as to remove pesticide residues. Among the literally hundreds of other uses are agents in gas masks, pollution control devices such as car catalytic converters and flue gas desulfurization.
Referring now to the chemistry of activated carbon, it should be noted that activated carbon has an extraordinarily large surface area and pore volume that gives it a unique adsorption capacity. Some material includes particles having surface areas as high as 5,000 m2/g. The specific mode of action is extremely complex, and has been the subject of much study and debate. Activated carbon has both chemical and physical effects on substances where it is used as a treatment agent. Activity can be separated into: (1) adsorption; (2) mechanical filtration; (3) ion exchange; and (4) surface oxidation.
Adsorption is the most studied of these properties in activated carbon. This action can be either physical or chemical in nature, and frequently involves both. Physical adsorption involves the attraction by electrical charge differences between the adsorbent and the adsorbate. Chemical adsorption is the product of a reaction between the adsorbent and the adsorbate. Adsorption capacity depends upon: (1) physical and chemical characteristics of the adsorbent (carbon); (2) physical and chemical characteristics of the adsorbate; (3) concentration of the adsorbate in liquid solution; (4) characteristics of the liquid phase (e.g., pH, temperature); and (5) amount of time the adsorbate is in contact with the adsorbent (residence time).
Mechanical filtration involves the physical separation of suspended solids from a liquid passing through carbon arrayed as a porous media in a column or bed. Any finely divided solid, such as sand or cellulose, can accomplish this. While this accounts for some of the clarification properties of carbon, it is seldom the sole reason for the selection of carbon as a clarification medium. The effectiveness of filtration depends on particle size, bulk density, and hardness. While a smaller particle size results in a clearer liquid, it also slows the speed of processing. Bulk density determines how much carbon can be contained in a given container. Hardness is relevant, since the particles need to have sufficient strength to block the particulate matter being filtered.
Coal is a natural ion exchanger, and ion exchange can be enhanced by chemical activation. Carbon surfaces have both negative (anionic) or positive (cationic) charges to attract free ions in solution or suspension, depending on how they are treated. Treatment of carbon with a base increases the capacity of carbon to exchange anions; acidulation of the surface makes carbon a powerful cation exchanger. Surface oxidation involves the “chemisorption” (chemical adsorption) of atmospheric oxygen to the carbon and the further reaction of the surface oxides that chemically react with other substances that are oxidized. The surface of activated carbon has an electrical double layer.
The purity of the carbon and other substances found with it depends on the source, the manufacturing process, whether it is a virgin or regenerated source, and formulation. Bone char is generally 9-10% carbon and about 90% ash, with 80% of bone char composed of calcium phosphates. Activated carbon can be combined with a number of other substances that are effective agents for ion exchange. These might include filtering aids, e.g., silicon dioxide, and resins. The carbon is usually packed in a column that is non-reactive, but sometimes columns and other packing material will also provide ion exchange activity. Some of these are ceramic or polymeric. Activated carbon may also be used with a variety of metal catalysts, including nickel, copper, palladium, ruthenium, and titanium. Chlorine is often used with activated carbon to remove phenols and other chemicals.
Carbon can be reused if the adsorbed substances are removed. This process is known as ‘regeneration’. Simply heating the spent carbon at a given temperature for an adequate length of time can regenerate activated carbon to the point where it can be reused for tertiary wastewater treatment (thermal regeneration). Thermal regeneration inevitably results in the loss of carbon. Also, thermal methods may not be the most efficient, inexpensive, or reliable method, so a number of solvents, acids, and alkalis may be employed to remove the adsorbed substances. These include such things as carbon tetrachloride, hydrochloric acid, hydrogen peroxide, potassium hydroxide, sodium hydroxide. Optimization of the regeneration process depends on the substances adsorbed as well as the structure of the activated carbon.
Charcoal dates back to the prehistoric discovery of fire. Ancient Hindus filtered their water with charcoal and Scheele discovered the fact that certain types of charcoal had adsorptive capacity, i.e., were chemically ‘active’ in 1773. Charcoal was found to decolor tartaric acid in 1785. In 1794, charcoal was first applied to the refinement of sugar. Natural forms of activated carbon such as charred animal bones (bone black) were used to refine sugar. Inventors patented a number of methods to improve the clarification, decolorization, and purification power of the bone char. These included improvements in: the control of the heat of carbonization; differential oxidation; mixing of bone with anthracite or bitumenous coal; addition of calcium phosphate to carbonized sugar; the packing of various clays upon the bone char in the retorts; complexing with various binders; and acidulation. By 1901, scientists had developed ways to synthesize activated carbon from coal that had equivalent or superior adsorptive and decolorizing capacity to bone black.
According to the prior art, most carbon material can be used to make activated carbon and the academic literature contains many references to activated carbon derived from many agricultural and industrial high-carbon waste products. Commercial activated carbon, however, is manufactured from only a few carbon sources: wood and sawdust, peat, coal, oil products, and nut shells and pits. Wood products and low-grade coal have some original porosity and are easier to activate than dense materials such as anthracite. However, any high carbon material can be activated, and it is generally not possible to discern the original starting material of an activated carbon product.
Activated carbon manufacturing consists of a charring or carbonization step in which most of the non-carbon material (and much of the carbon) is volatilized by pyrolysis (usually between 500 and 750° C.). The weight loss is usually 60% to 70% and much CO2 is volatilized. Coal is usually first pre-oxidized at 150° to 250° C. to prevent the coal from becoming thermoplastic during charring and collapsing the pore structure. The fine pore structure is formed in an activation process. In gas activation, an oxidizing gas, such as CO2 is used at a high temperature to erode pores into the char. In chemical activation, the char is impregnated with a chemical and then fired to high temperatures (usually 800 to 1000° C.). The activating chemical corrodes the carbon to form the pore structure. Chemical activation also alters the carbon surface. Activation chemicals are usually strong acids, bases or corrosives (phosphoric acid, sulfuric acid, KOH, zinc chloride, potassium sulfide, or potassium thiocyanate). After activation, the chemicals are washed out for re-use, and the final pore structure depends on the nature of the starting material and the activation process. Materials with an original pore structure like wood take less processing than more dense and isotropic material like coal or tar. Impurity amounts are usually higher in the less carbon dense materials, however.
The surface chemistry of the activated carbon is strongly influenced by the activation process and subsequent chemical treatment. The surface contains abundant oxygen and hydrogen groups, which can decompose to CO2 and water. Other surface oxide complexes that have been found include phenols, carbonyl, lactone, carboxylic acid, and quinones. The abundance of surface complexes causes activated carbon to be a good absorber of many gases and aqueous chemicals. The non-selective absorption of many chemicals makes activated carbon an excellent absorber in poisoning or environmental contamination. Non-selectivity is less desirable when a specific chemical is to be removed from a process stream. Activated charcoal is impregnated with potassium carbonate for efficient catalytic reduction of CO2 gas, and debittering of citrus peels is mostly accomplished throughout the use of ion exchange resin.
Charcoal is generally considered to be a natural agricultural product. Both charcoal and carbon black form naturally (forest fires), and have been used by man for thousands of years. Activated carbon does not occur naturally. A highly controlled two- or three-stage process is needed to form the high porosity of activated carbon. The activation step also requires either the addition of a synthetic chemical or direct injection of CO2 or O2 during the activation firing. Highly porous activated carbon should be considered synthetic. Bone char results from the destructive distillation of animal bones, and bone char production does not include an activation step. It is more analogous to the leftover material in the destructive distillation of coal to make coal tar.
According to the prior art, the micronization, activation and preparation of carbonaceous materials is known in the art. For example, see U.S. Pat. No. 6,318,649 to Mazurkiewicz; U.S. Pat. No. 6,425,941 to Roodman; U.S. Pat. No. 4,905,918 to Selles et al.; U.S. Pat. No. 5,174,512 to Orlandi; U.S. Pat. No. 5,575,824 to Brown et al.; U.S. Pat. No. 5,732,894 to Sheahan; U.S. Pat. No. 4,045,092 to Keller; U.S. Pat. No. 4,921,831 to Nakai et al.; U.S. Pat. No. 5,151,173 to Vaughn et al.; U.S. Pat. No. 5,880,061 to Yoshino et al.; U.S. Pat. No. 5,071,820 to Quinn et al.; and U.S. Pat. No. 6,064,560 to Hirahara et al. However, there is considerable room in the art for the exploration of additional and beneficial uses of micronized or nanoized, and activated, anthracite as a precursor for further utilization and/or functionalization. Also, according to the prior art, anthracite has been used as a precursor for the production of activated carbon. However, the physical dimension of the anthracite particles used has been in a mesh range with little or no consideration to particle size distribution. The smallest mesh size of carbon to be used for activation referenced in the prior art is 100 mesh (approximately 150 micrometers), and its corresponding minima/maxima range or maxima-only range defined in the greater than 100 micron region.